FIG. 21 illustrates a conventional laser cutting machine, which includes a laser oscillator 1 that radiates a laser beam 2. The beam is reflected from a mirror 3 in a predetermined direction onto a condenser 4, which gathers and focuses the laser beam 2. A workpiece 5 is set in the focusing position of the condenser 4 and is supported by a support 6. A cutting head 7, which includes the condenser 4, also has a nozzle 8 at its front end. A profile sensor 9 detects a distance between the condenser 4 and workpiece 5, while a solenoid 10 raises and lowers the profile sensor 9. An x-axis motor 11 moves the workpiece 5 in an X axis direction, a Y-axis motor 12 moves the workpiece 5 in a Y axis direction and a Z-axis motor 13 moves the cutting head 7 in a Z axis direction. A cutting table 14 is moved in the X and Y directions by the X-axis motor 11 and Y-axis motor 12. A control box 15 controls the cutting table 14, the cutting head 7 and the laser oscillator 1. A numerical controller 16 (hereinafter referred to as the "NC unit"), is contained in the control box 15. This unit comprises a central processing unit (CPU) 17, a ROM 18 for storing a system program that controls the NC unit 16 and a RAM 19 for storing cutting programs, parameters and other various data. The NC unit 16 also includes an interface 20 for transferring analog and digital signals to and from external devices, a graphic control circuit 21 (including servo amplifiers equivalent to the interfaces to the motors) for converting data processed by the CPU 17 into display signals, and display 22, such as a CRT or a liquid crystal display. A key switch unit 23 comprises various key switches, the signals from which are input to the NC unit 16. A relay unit 24 is switched on/off by a signal output from the NC unit 16. A switch 25 performs an automatic power shut-off validation function. An earth leakage circuit breaker 26 with a test lead terminal (hereinafter referred to as the "main breaker") delivers power to the laser cutting machine, and a relay 27 with normally closed contacts is connected to the test lead terminal of the main breaker 26. The test lead terminal is used to check by means of an external signal that the earth leakage circuit breaker is operative when an earth leakage occurs. A tactile sensor is installed in the periphery of the cutting head 7. The contact type sensor, illustrated as the profile sensor 9 (FIG. 21) makes contact with the surface of the workpiece 5 and moves vertically according to the unevenness of the surface of the workpiece 5. Another sensor, such as a capacitance sensor or an optical sensor, may be substituted for the profile sensor 9.
The operation of the laser cutting machine will now be described. By switching on the main breaker 26, the laser cutting machine is fed with electricity and is ready for operation. A command from the control box 15 causes the laser oscillator 1 to radiate the laser beam 2. The laser beam 2 is reflected by the mirror 3 onto the condenser 4. The laser beam 2 is gathered by the condenser 4 into a minute spot diameter (the position where the beam is gathered is referred to as the "focus position").
The distance between the workpiece 5 and condenser 4 is detected by the profile sensor 9, which generates a corresponding electrical signal (hereinafter referred to as the "focus distance detection voltage"). The control box 15 compares a set position voltage and the focus distance detection voltage, and drives the Z-axis motor 13 and moves the cutting head 7 vertically to maintain a set distance between the workpiece and cutting head. This control causes the laser beam 2 to focus the beam on a spot on the workpiece 5, thereby concentrating a large amount of energy on one point and forming a hole in the workpiece 5. This workpiece 5 can be cut into an optional shape by using the X-axis motor 11 and Y-axis motor 12 to move it according to commands from the control box 15.
The laser cutting machine is capable of non-contact cutting, as described above, and does not generate large noise as is produced by a press. The laser is also suited for night-time unmanned automatic operation.
An example of a night-time untended automatic operation procedure for the laser cutting machine will now be described in accordance with FIGS. 22 and 23. Before going home, an operator sets a relatively large work piece 5 on the cutting table 13 to be cut during the night. The operator then calls from the NC unit 16 a cutting program Pr(0) having a multiplicity of product cutting programs Pr(1) to Pr(n) as subprograms (see FIGS. 22(a) and (b)). The operator also validates the automatic power shut-off switch 25 on the control box 14 and sets the automatic operation start switch (not shown) to "ON". After making sure that automatic operation has started, the operator goes home.
The laser cutting machine operates in accordance with commands registered in the cutting program Pr(0) which consists of steps S501 to S562 (FIG. 22(a)) and automatically turns out a multiplicity of products during the night. A program end code M30, registered at the end of the cutting program Pr(0), is executed to stop the automatic operation. When the program end code M30 is executed, the automatic power shut-off function initiates its operation. The automatic power shut-off function automatically performs the operator's procedure of stopping the laser cutting machine in accordance with a program (automatic power shut-off means) stored beforehand in the memory of the NC unit 16. When the relay 27 is finally switched ON by the command from the NC unit 16, the normally closed contacts of the relay 27 open. The normally closed contacts of the relay 27 are connected to the test lead terminal of the main breaker 26. When the normally closed contacts of the relay 27 open, the main breaker 26 operates to shut the power off, stopping the power supply to the laser cutting machine. This reduces the power consumed by the system.
Coming to the job site the next morning, the operator removes the N finished pieces from the cutting table 13, thereby terminating the night unmanned automatic operation. FIG. 22(b) indicates plural products A of rectangular shape and the arrows represent cutting directions. If a fault occurs during the night-time unattended automatic operation, the control box 15 detects the fault and stops the system.
An exemplary cutting operation wherein the cutting program Pr(0) is executed without fault will now be described with reference to FIGS. 22(a) and 22(b). The commands in the cutting program Pr(0) are assumed to be as follows: moving the cutting head from a cutting program starting position P0 to a cutting starting position P1 (step S501); calling and executing the subprogram Pr(1) (step S502); moving to a next cutting starting position P2 (step S503); calling and executing the subprogram Pr(2) (step S504); repeating the calling, executing, and moving operations until the final subprogram Pr(n) is executed (step S550); moving the cutting head to a cutting end position Pend (step S561); executing the program end code M30 (step S562); and terminating the cutting program Pr(0). The above operation results in N finished pieces
An exemplary cutting operation is illustrated in FIG. 23(a) and 23(b), wherein a fault occurs in the subprogram Pr(m) during the execution of the cutting program Pr(0). Although the cutting program Pr(0) is run without fault from the subprogram Pr(1) to subprogram Pr(m-1), cutting is stopped due to a fault somewhere between the subprogram Pr(m) and subprogram Pr(n). For this reason, the number of products turned out equals (M-1) which is less than N. The fault occurs at position Ps in FIG. 23(b).
The conventional laser cutting machine is arranged as described above. If a fault takes place, for example, during night-time unmanned operation, the control box 15 detects the fault and stops the system, terminating cutting in an unfinished state. Hence, the unfinished products must be cut the next day. FIG. 24 illustrates a processing flowchart for resuming the cutting processing procedure at a time when a faulty cutting condition occurs during automatic operation and a cutting stop command is given by the control box 15. However, in FIG. 24, cutting is resumed at a position different from the cutting stop position.
When the laser cutting is stopped due to a fault or the like in step 10, the operator examines a cutting error or the fault factors and takes appropriate corrective action. Next, the operator searches for a block in the NC program where cutting is to be resumed in step 11, moves the cutting head 7 to the resumption position corresponding to the resumption block in step 12, and restarts cutting at that position in step 13. Often the cutting operation is stopped by a signal from the profile sensor 9 due to a fault, in which the tip of the profile sensor 9 is welded to the workpiece 5. This welding effect results from spattering or the like produced during piercing (drilling at a cutting start) or other work. When this welding fault occurs, it is necessary to determine whether the profile sensor 9 is normal or not before restarting the system.
FIG. 25 illustrates a processing flowchart used to judge whether the fault which has occurred is the welding of the profile sensor 9. Referring to FIG. 25, if a profiling fault is detected in the signal from the profile sensor 9 in step 570, the solenoid 10 is switched on in step 571 to attempt to raise the profile sensor 9. Once the solenoid 10 attempts to raise the sensor 9, a detection voltage Vf representing the position of the profile sensor 9 is compared with a judgement reference voltage Vh in step 572. The judgement reference voltage Vh employed is a voltage which should be exceeded by the profile sensor 9 detection voltage when the profile sensor 9 is raised ordinarily. Accordingly, if Vf&gt;Vh, the profile sensor 8 has been raised normally. In this case, processing under a normal condition is performed in step 573. Conversely, if Vf.ltoreq.Vh, the system determines that the profile sensor 9 has not been raised ordinarily for some reason, and fault processing is carried out in step 574.
In the known laser cutting machine, when this processing is carried out, the operator must perform complicated and troublesome operations until cutting is resumed. In addition, if a fault occurs during unmanned automatic operation, e.g., night-time operation, cutting remains stopped until the user arrives the next day upsetting a production schedule.